One of Earth's closest neighbours, Mars, is still at least 56 million km away - so what would it take to put a human on the Red Planet? The BBC asked scientists from Imperial College London to design a mission which could land crew on Mars and get them back safely.

They came up with a spinning spacecraft to create artificial gravity on the nine-month trip, combating bone loss and muscle wastage. A shield would be needed to protect against radiation from the Sun and cosmic rays. A robot sent ahead of the mission would generate fuel from the Martian ice for the return journey.

Imperial College physicist Martin Archer answered your questions about the Red Planet in a Twitter Q&A.

Here is an edited version of the session which was held on Wednesday 24 July 2013.

Question from @mobzi_malone: What's the fastest a spacecraft can travel with people in? Is 9 months the shortest journey time? #BBCMars

Martin answers: 9 months is the time it takes if you let gravity do most of the work so you don't need as much fuel #BBCMars

Question from @SJSumner: How fast (rpm) would the tethered landing and habitat modules be rotating to create an earthlike gravity? #BBCMars

Martin answers: Because of our 60m tether we only need a 3rpm (20s rotation period) to simulate Earth gravity. #BBCMars

Question from Jav Hidalgo via email: I was just wondering if you thought that a crew of three is enough? #BBCMars

Martin answers: 3 people is based on the ISS expeditions, though there are 2 at a time. With more people you need more resources on the way! #BBCMars

Question from @A_M_Swallow: How are you going to power the ground installations? Nuclear? #BBCMars